In semiconductor chip technology, EUV lithography is the next-generation lithography exposure apparatus for generating chip structures in the range of ≦32 nm. The quality of the lithographic imaging is determined by an illumination system for the homogeneous illumination of a mask (reticle) and an optical imaging system for transferring the mask structure to the wafer.
The illumination system contains the actual source module 1 which—as is shown in FIG. 2—usually has a plasma generation arrangement 2 for ionization of gas for the plasma 3 which, e.g., emits a radiation in the EUV spectral region, a debris filter 4, and a radiation collector 5. The collector 5 images the primary source location of the plasma 3 inside the source module 1 in an intermediate focus 61 forming a secondary source location, as it is called, in the intermediate focus aperture 62 at the output of the vacuum chamber 11 of the source module 1. Downstream of the secondary source location (intermediate focus 61) in the direction of a lithographic illumination system, the radiation should be free of debris and spectrally filtered.
Usually, arrangements of reflectors 51 which are nested one inside the other in a rotationally symmetric manner are used for focusing the highly divergent radiation from the plasma 3. The reflectors 51 are usually thin nickel plates which are coated on the inner side with suitably reflecting metal layers (e.g., molybdenum, ruthenium, rhodium, palladium, etc.). All of the reflector shells reflect in grazing incidence at incident angles of <15°. With large aperture angles in particular, optically advantageous collectors 5 based on a Wolter design are used (see FIG. 2a). Multiple reflections on nested multi-shell reflectors 51 are used for focusing, as is shown in FIG. 2a, as a combination of hyperboloids of revolution 52 and ellipsoids of revolution 53.
For production-ready equipment for semiconductor lithography, there are certain defined specifications in the intermediate focus 61 (secondary source location, intermediate focus aperture 62 or output opening of the source module). Apart from the required far-field intensity distributions based on the required homogeneous illumination in the reticle plane, possibly the most important requirement is that the EUV radiation output in the intermediate focus be greater than 115 W at a given etendue.
The etendue (radiant flux) is a predetermined fixed value for the entirety of the optical system. For future EUV lithography scanners, etendue values of 3.3 sr·mm2 are being discussed. This value is given by specific vignetting (through apertures or the like) existing in the beam path and determines, along the light path, what the size of the diameter of the beam bundle may be and what the maximum angles of the beams relative to the optical axis may be without certain portions of the beams being shadowed by any apertures. As a good approximation for small angles relative to the optical axis, it may be estimated that the square of the effective diameter of the luminous region multiplied by the solid angle of the collector with respect to the source point is approximately equal to the square of the diameter of the aperture in the intermediate focus multiplied by the solid angle of the radiation in direction of the intermediate focus.
Accordingly, the situation at the collector is as follows: With a solid angle of the collector relative to the emitting plasma (primary source location) of 3.14 sr (π-collector, aperture angle 60°) and a solid angle of the radiation in direction of the intermediate focus (secondary source location) in the range of 0.03 sr to 0.2 sr (depending on the increase in the collector), there are exact calculations with an aperture opening of about 10 mm in the plane of the secondary source location, a maximum permissible diameter of the emitting source region <1 . . . 1.3 mm, and maximum lengths in axial direction of <1 . . . 1.6 mm. Additional radiation losses occur for larger dimensions so that considerable efforts must be directed toward a spatially strictly limited and very dense plasma.
With required radiation outputs in excess of 115 W in the intermediate focus, the collector is typically acted upon by a total radiation output of >20 kW. Assuming an average reflection factor of 50%, then 50% of the radiation impinging on the reflecting surfaces is absorbed in the collector. Given that the end faces of the nested collector shells are optically dull, a considerable portion of radiation is additionally absorbed at these end faces so that the incident radiation output is dissipated by the collector on the whole chiefly as heat output. The high thermal loading of the collector causes distortions in the far-field intensity distribution and, therefore, degradation of the homogeneity of the far-field intensity distributions behind the intermediate focus. Further, the arrangement of a cooled collector is complicated because cooling pipes must be arranged in the radiation shadows to prevent further radiation losses through shadowing.
Manufacture of the collector is very complicated on the whole and is consequently extremely expensive. In order to prolong the life of the collector optics, they must be protected by corresponding debris filters against fast ions from the plasma and sprayed electrode material. Since collectors customarily have diameters of ˜500 mm, the debris filters must likewise have large dimensions. Debris filters are a filigree construction of many specially arranged plates (see, e.g., DE 102 37 901 B3 or DE 10 2005 020 521 A1) which are likewise very expensive to produce. Another disadvantage of the large collector optics is the required vacuum chamber with dimensions of approximately 1 m in diameter and 1.5 to 2 m in length. Apart from high costs, this causes long pumping times above all.
The essential parameter for the source module in its entirety, comprising the plasma generation arrangement, debris filters and collector, is the collection efficiency, as it is called. This expresses as a percentage how much of the radiation output generated in the solid angle 2π arrives behind the intermediate focus aperture. Currently, collection efficiencies of 1.5% to 3% are achieved in so-called α-tool sources. Accordingly, this collection efficiency is disproportionately low compared to the enormous constructional and monetary resources required.